4 Answers

I guess that would be waiting until you know for sure you can afford a family. 

You can start a family any time you want. There is no waiting period.

40 weeks from your last period to birth- or 36 weeks from your 1st missed period (although you can still spot)  38 weeks from conception to birth (assuming you have regular periods that are 4 weeks apart)....these are all "norms" used by the medical community.  after your first ultra sound they will tell you how pregnant you are and give a close estimate for your delivery date.  It's best to take prenatal vitamins and to stop smoking, drinking, or doing drugs for several months before you try to get pregnant. (ask you doc about any Rx meds you are taking too)

This may have social and cultural guidelines, in the US, there seem to be no guidelines.  Not to say that there should not be some social norms..